I have the same problem on my 95 SL. Started immediately after I pulled the gauge cluster to replace some bulbs and disconnected the cluster harness to completely remove the cluster.

I have heard that a bad seatbelt switch can cause the light to come on, but your best bet is to pull codes. That's what I will eventually do when I get time...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car